This invention relates to a copy-repeater arrangement and to a method of adjusting the same.
The present invention is applicable with respect to any structure wherein a pair of elements are to be rotated synchronously but must be first angularly adjusted relative to one another, to assume a predetermined relative angular position. Since the invention is especially useful in copy-repeating apparatus, such as scanners, facsimile apparatus and clichegraphs, it will be described with reference thereto in the description which follows.
In photogravure printing it is necessary to prepare rotary printing forms which are then used to print onto paper or the like. To do this, the original to be printed is usually electro-optically scanned and the signals thereby derived are used to actuate an engraving device which engraves onto a rotating printing form, such as a printing cylinder, a series of depressions or "cups" which are distributed in accordance with a screen point system or "raster". The scanning is carried out by rotating a scanning drum in synchronism with the printing form; the scanning drum carries the original which is scanned by a scanning head that is capable of moving in parallel with the drum axis. The tone values of the original which are scanned in accordance with the screen used on the recording side are converted into analog electrical signals which are used to control the movement of the aforementioned engraving device that is movable parallel to the axis of the printing form and has a tool that forms depressions therein. The depth of these depressions varies in dependence upon the tone value indicated by a particular signal. It will be understood that the signals indicative of the tone values scanned have a constant-frequency screen signal superimposed upon them in order to obtain the desired screen point distribution.
When the printing form is finished it can be used to print the desired image onto paper or the like. For this purpose it is inked so that the ink becomes retained in the depressions formed as outlined above. Since the depressions are of varying depth, it follows that they will accept varying quantites of ink and this, in turn, will result in printing of ink dots of varying color density, thus reproducing the tone value of the corresponding point on the previously scanned original.
The scanning drum and the printing form, hereafter called the scanning cylinder and the printing cylinder, of prior art machines of this type are each driven in rotation by a separate synchronous motor which is supplied either with net current or from a rectifier.
It is absolutely essential that both of these cylinders rotate in absolute synchronism, as otherwise there will be inaccuracies in the recorded (engraved) image. This is of even greater importance if color printing is to be carried out, because in that case a separate printing cylinder must be produced from the original for each color to be printed. Each printing cylinder of such a set then prints the image in a different color, and each image is printed over the preceding different-colored image, until the combined images afford the desired color effect. It is readily understandable that if there are any inaccuracies in the engraved images on one or more of the printing cylinders, the several colors of the different images will not precisely overlap, with the result that the finished color print is of very noticeably poor quality.
The aforementioned inaccuracies need not solely be the result of difficulties in the engraving of the image on the printing form; they can also result from improper synchronism in the rotation of the several printing cylinders during the actual printing operation. However, whereas this latter problem can be corrected by appropriate adjustments to the printing machine, inaccuracies which have resulted from improper locating of the engraved image on a printing cylinder, cannot be so corrected. Because of this, the printing cylinders must be engraved with great precision, i.e. the proper location of the engraved image or portions thereof on the cylinder must be assured to within e.g. about 1/100 mm.
These problems are aggravated by other requirements in some instances. For example, it is frequently required that on all printing cylinders of a cooperating set the engraved image should begin at a certain point or mark on the cylinder, such as a groove, which is present on all cylinders of the set at identical locations relative to the cylinder circumference. This requirement is made because it aids in later adjustments of the printing machine, and it evidently presents additional difficulties with respect to the proper positioning of the engraved image.
Also, it is often necessary that the engraving of a printing cylinder be carried out in two or more stages. For example, when the printing cylinder is to be used for printing one or more pages of a magazine where the page make-up may consist of text and/or pictures and advertisements, such as multi-stage engraving operation may have to be carried out. The advertiser is usually furnished so-called "proofs" of his advertisement, i.e. actual printing runs which show the quality of the advertisement reproduction, so that he may approve or disapprove of the quality. This means that the printing cylinder must be engraved with the advertisement in order to make it possible to print such proofs. The text and/or accompanying pictures which go on the same page as the advertisement may, however, be readied only very much later, just shortly before the magazine goes to press. The engraving of this material must therefore be carried out in a second engraving stage, and the material must be engraved with great precision in precisely the portions of the printing cylinder surface which have been reserved for this purpose. Moreover, this precision is required not only for a single printing cylinder when a set of color-printing cylinders is concerned, but for all cylinders of the set, and there must not be any deviation in the location of the added material from cylinder to cylinder of the set.
The known engraving equipment of the type under discussion does not provide for any association between the rotation of the cylinders and the initiation of the engraving operation; after the equipment has been started, the scanning cylinder and the printing cylinder rotate in synchronism but are angularly (i.e. circumferentially) offset with reference to one another. There is, therefore, no way in which the problems outlined above can be solved with such equipment unless auxiliary means are used which make it possible to turn one of the two cylinders involved in the engraving operation (i.e. the scanning cylinder and the recording cylinder) with reference to the other, to thereby permit the engraving to begin at a predetermined point on the printing cylinder circumference or, alternately, to restore the relative positions of the original for the first gravure on the scanning cylinder and the first gravure on the printing cylinder which existed at the time the first gravure was made.
The art does, indeed, teach a device which can be used for such a purpose. Each of the two cylinders is associated with a stationary signal generator which scans the cylinder and, when it detects a mark thereon, furnishes a signal indicate of the circumferential (angular) position of the associated cylinder. The signals derived from the signal generators of both cylinders are fed to and made visible on, the screen of a storage oscilloscope.
The reluctance motor driving one of the cylinders is connected directly to net voltage, whereas an induction voltage regulator is connected between the net and the reluctance motor driving the other cylinder. An induction voltage regulator is a net-voltage operated induction machine having a rotor which can be only manually turned relative to the associated stator, making it possible to phase-shift the voltage induced in the rotor with reference to the net voltage. Therefore, turning of the rotor in the prior-art arrangement makes it possible to obtain a turning of one motor (and its associated cylinder) with reference to the other motor and the cylinder driven by the same. To do so, an operator turns the rotor of the regulator while watching the oscilloscope screen; when the two visually reproduced signals coincide on the screen, the cylinders have the desired relative angular orientation.
However, while this suggestion constitutes a definite advantage over the approaches tried prior to it, there are certain disadvantages involved.
One of these resides in the fact that the coincidence setting must under all circumstances be carried out manually. This requires not merely the presence of an operator, but the presence of a skilled and experienced operator. Even then, the accuracy of the coincidence obtained may vary widely from case to case, depending upon skill of the operator, his attention or lack of it, possible distractions, and similar factors.
Another disadvantage is that the entire apparent power for the motor is supplied via the voltage regulator. Since reluctance motors have high idle power and the dissipation losses of an induction voltage regulator are substantial, the voltage regulator must be constructed for a high throughput capacity, which makes it expensive.
Finally, the use of a storage oscilloscope and a rotary transformer make this prior art arrangement very expensive.